1. Field of the Invention
This invention generally relates to optical cables and, more particularly, to a method and fixture for testing fiber optical plug/jack connector microlenses.
2. Description of the Related Art
Conventionally, the fiber endfaces (optical interfaces) of two optical fiber connectors are pressed together, resulting in a direct glass to glass or plastic to plastic, contact. The avoidance of glass-to-air or plastic-to-air interfaces is critical, as an air interface results in higher connector losses. However, the tight tolerances needed to eliminate an air interface make these connectors relatively expensive to manufacture.
FIG. 1 illustrate the assembly of the components of a Transmission Optical SubAssembly (TOSA) to form a light transmitting/receiving structure (prior art). The TOSA 10 of FIG. 1A includes an optical package 20, a cylindrical holding barrel 26 and a ferrule 30. The optical package 20 has an optical or optoelectronic component 21.
FIG. 2 is a partial cross-sectional view of a TOSA optical cable plug with lens (prior art). The plug 100 is made from a plastic housing 102 with a bored ferrule 106 to secure an optical fiber 108. The plug 100 also includes a plastic lens 110, manufactured as a subassembly, integrated into the plug. The lens 110 has a curved surface to create a focal plane where the plug mates with a jack. The advantage of using a lens includes the ability to focus light on a point, such as a photodiode or optical fiber core face, while transceiving light in a collimated beam between connectors. The gain resulting from the use of a lens permits a low loss air gap to be formed between the plug and a connecting jack. However, the focusing of light on a fiber core face requires that the fiber core and microlens be properly aligned. In addition to the expense of manufacturing a 2-part plug, the plug must be made to relatively tight tolerances, so that the lens focal plane aligns with the jack, which also increases the cost of the plug.
In conventional manufacturing processes, components of a light transmitting/receiving structure have inherent flaws that frequently cause deviations in the optical signal transmission path of the structure after being assembled. The light transmitting/receiving structure thus produced is of poor quality. With respect to the TOSA of FIG. 1, possible causes of deviations in the optical signal transmission path include misalignment of the optical axis of the optical or optoelectronic component 21, improper location of a focusing element 110 (FIG. 2), loose or slack connection between the optical package 20 and the holding barrel 26, loose connection between the holding barrel 26 and the ferrule 30, failure to firmly grasp the optical fiber that is inserted into the axial hole, inadequate core concentricity of the fiber stop 32, and failure of the ferrule 30 to firmly grasp the fiber stop 32.
One method of testing an optical cable involves connecting the cable between an optical transmitter and receiving circuitry, and measuring optical communication errors. As noted in US 2005/0184227 (Kuu et al.), another method involves clamping a TOSA into a test jig and measuring optical signal strength.
In the field of optics, the “sharpness” of a lens is typically a primary concern. Spherical aberration affects the whole image captured by a lens, while astigmatism and coma mostly affect the edges and corners. The tests for these types of problems can be extensive. For example, the USAF 1951 lens test chart and its variants are a standard unsuited for computer analysis because of its fragmented arrangement, and poorly suited for visually estimating MTF (Modulation Transfer Function), as a person must strain their eyes to find highest resolution at which bars can be distinguished. Improved versions, such as the Koren test chart, permit a tester to state that at a given f-stop, the center or corner of this lens has 50% MTF at roughly x lp/mm and 10% MTF at y lp/mm. These numbers are well standardized and closely related to perceived image quality and resolution. However, a complicated lens sharpness test is not an efficient means of determining lens alignment, especially if sharpness is not a critical issue.
It would be advantageous if the lens alignment of an optical connector plug or jack could be easily tested.